feature - modernize InQL for v0.3 showcase (#25)

[dannymeijer]

Summary

This PR modernizes InQL #25 for the Incan v0.3 RC2 toolchain and turns the branch into a stronger InQL showcase. It updates the scalar expression surface around union-backed literals and structured lowering errors, adopts additional v0.3 language features where they improve the package shape, refreshes tests/docs/examples for the current API shape, regenerates the lockfile, and adds an ignore rule so local agent state under .agents/state/ stays out of public PRs.

The branch now passes end-to-end with the local Incan main-worker fix for incan#617, including the pub::inql consumer smoke check for alias-backed public helper exports.

Type of change

• Bug fix
• New feature
• Refactor / maintenance
• Documentation
• CI / tooling
• RFC (adds/updates docs/rfcs/*)

Area(s)

Select the primary areas touched (labels sync from checked lines when the triage workflow runs):

• Package & tests
• Specification (RFCs)
• Documentation
• Automation & repo config
• Other

Key details

• User-facing behavior: InQL examples and docs now show the v0.3-native API shape, including lit(...), builder-backed filters/projections/aggregates, Session execution paths, and current DataFusion-backed behavior.
• v0.3 showcase usage: The branch now uses union-backed lit(...), match over union values, structured Result[..., SubstraitLoweringError] fallible lowering paths, Result.map_err(...), tuple-target comprehensions, if let for optional proto unwrapping, value enums for stable string-backed categories, native u32 Substrait anchors, and public symbol aliases for helper forwarding.
• Risks: This is a broad package/docs/test refresh across the InQL surface. The main risks are API expectation drift in downstream examples and subtle Substrait output-shape regressions.

Testing / verification

• make ci (or make fmt-check, make build, make test)
• Manual verification described below

Manual verification notes:

• /Users/danny/Development/encero/incan/target/debug/incan --version reports incan 0.3.0-rc2 after rebuilding the main Incan worker at 8003bdad.
• make ci INCAN=/Users/danny/Development/encero/incan/target/debug/incan passes fmt-check, test-style, library build, all 88 InQL tests, and the pub::inql consumer smoke check.
• The minimized dependency repro from incan#617 also passes with the same compiler build.
• Before the final public-alias adoption, make build-locked INCAN=/Users/danny/Development/encero/incan/target/debug/incan and make test-locked INCAN=/Users/danny/Development/encero/incan/target/debug/incan passed after regenerating incan.lock with RC2.
• git diff --check HEAD passed.

Docs impact

• No docs changes needed
• Docs updated
• Docs follow Divio intent (tutorial/how-to/reference/explanation) where applicable

If docs updated:

• Link(s): docs/architecture.md, docs/language/explanation/*, docs/language/reference/*, docs/rfcs/*, docs/release_notes/v0_1.md, examples/*

Checklist

• I kept public docs user-focused and moved internals to contributing docs when appropriate
• I avoided duplicating canonical install/run instructions in multiple places
• I added/updated tests where it materially reduces regressions

Closes #25